There is an increasing demand for high quality premium coke for the manufacture of large graphite electrodes for use in electric arc furnaces employed in the steel industry. A number of properties are of importance in characterizing the quality of graphite electrodes. One such property is density. Usually the higher the density the better the electrode. Another important property relating to premium coke is the crush resistance of the coke. The handling and processing of coke particles necessary for the manufacture of electrodes is enhanced by increasing crush resistance. This property of the coke is also a factor in the strength of the electrodes. Users of premium coke continuously seek graphite materials having higher densities. Even a small change in density can have a substantial effect on large electrode properties. Still another property which is important in characterizing the quality of premium coke is CTE, which may vary from near zero to as high as plus eight centimeters per centimeter per degree centigrade.times.10.sup.-7. Users of premium coke also seek materials having a low CTE value.
Premium coke is manufactured by delayed coking in which heavy hydrocarbon feedstocks are converted to coke and lighter hydrocarbon products. In the process the heavy hydrocarbon feedstock is heated rapidly to cracking temperatures and is fed into a coke drum. The heated feed soaks in the drum in its contained heat which is sufficient to convert it to coke and cracked vapors. The cracked vapors are taken overhead and fractionated with the fractionator bottoms being recycled to the feed if desired. The coke accumulates in the drum until the drum is filled with coke at which time the heated feed is diverted to another coke drum while the coke is removed from the filled drum. After removal the coke is calcined at elevated temperatures to remove volatile materials and to increase the carbon to hydrogen ratio of the coke.
In the manufacture of large graphite electrodes, calcined premium coke particles obtained from the delayed coking process are mixed with pitch and then baked at elevated temperatures to carbonize the pitch.
According to this invention, calcined coke having increased density is produced by contacting the coke prior to calcining with coker feedstock and thereafter carrying out the calcining operation. In one embodiment the process is employed to obtain premium coke of increased density for use in the manufacture of electrodes employed in the production of steel.